Here's the finished framework for the top of the cabin except for a little bit of dressing up that I plan to do Wednesday.
I removed the arch piece from the jig, then dressed it up with a belt sander. I used the pattern to notch it for the starboard cabin top stringer. I then clamped it in place to glue. Please note in the picture there is a extra stringer on the starboard side that runs from this arch piece forward to the front of the cabin. This is not in the original plans. I did this for a couple of reasons which I discuss below.
Looking around the web at various other Elvers e.g. Mona, Daniel G, Elver, and Gizmo, and an unnamed one, I noticed a variety of hatch configurations. Some of these had sliding hatches. One built in Alaska does not have any hatch in the top of the cabin. As far as I could see from the (sometimes small) pictures I did not see a forward hatch on any of the boats. Thus I have decided not to put one in for now.
The plans call for 1/4" plywood bent to shape over the cabin. They also mention doing it in sections if necessary. In order to do it using a single 4x8 sheet, I have decided to cut the plywood into 2' wide sections. I will glue and screw on the sections starting forward and working my way back. The extra starboard stringer will help the plywood bend to the correct shape so it will look good. It also looks more symetrical on the inside. If later I decide to make the forward hatch it should be an easy matter to cut it out and not have to deal with much distortion of the plywood.
As always, this is a matter of matching the plans to my situation rather than stating that I am "improving" them.